The parasympathetic nervous system regulates cardiac rate, conduction, and contractility through the actions of released acetylcholine on muscarinic receptors. The events linking muscarinic receptor occupation with these physiological responses are poorly defined Biochemical responses coupled to muscarinic receptors include increases in cyclic GMP synthesis, decreases in cyclic AMP formation, increases in K permeability and enhanced turnover of the membrane phospholipid phosphatidylinositol. Neither the relationship of these events to receptor binding nor their relationship to one another has been systematically examined in a single cardiac system. We propose to study these biochemical responses in dissociated cells from embryonic chick hearts. Muscarinic receptors in the intact dissociated cells will be characterized by radioligand binding studies, and differences in the receptor populations coupled to these responses will be analysed by comparing agonist and antagonist potencies and the effects of perturbations that alter receptor number and affinity. Conditions under which selected muscarinic responses are inhibited will be used to explore the unique requirements of each response and to ask whether the four responses studied are independent or sequential events. Additionally, changes occurring ontogenetically and with cell culture will be utilized to examine interrelationships between responses and to suggest which responses are linked to the chronotropic effects of muscarinic agents. The literature concerning physiological responses to muscarinic agents suggests that multiple receptor subtypes or multiple second messenger systems confer selectivity on these processes, i.e. that rate, contractility and conduction may be differentially controlled. Radioligand studies alone do not elucidate the processes between receptor occupation and response. An in-depth investigation into the biochemical responses that accompany muscarinic receptor occupation is necessary to understand how parasympathetic stimuli alter cardiac cell function ad how therapeutic interventions might be designed to recruit or block selected parasympathetic effects on themyocardium.